Process for producing a high strength and high toughness sinter

ABSTRACT

The high-strength and high-toughness sinter of the present invention includes a crystal agglomerate maintaining a fibrous shape and composed of crystals of SiC and MC 1-x  wherein M is Ti and/or Zr and x is a number of 0 or more but less than 1. This sinter is produced by laminating an inorganic fiber of a particular inorganic material containing titanium and/or zirconium molding the laminate into a predetermined shape, and conducting heat-sintering simultaneously with the molding or after the molding in an atmosphere of a vacuum, an inert gas, a reducing gas, or a hydrocarbon gas at a temperature of 1,700 to 2,200° C.

This is a division of application Ser. No. 07/265,254 filed Oct. 31,1988, now U.S. Pat. No. 4,990,470.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high-strength and high-toughnesssinter (ceramic composite material) and a process for producing thesame. The high-strength and high-toughness sinter according to thepresent invention is used mainly for applications such as members of aninternal combustion engine, e.g., a piston ring or an auxiliarycombustion chamber, and members of a rocket engine, e.g., a nose cone ora nozzle.

2. Description of the Prior Art

Ceramics having excellent heat resistance known to the art include,e.g., oxide ceramics such as Al₂ O ₃, B₄ O, MgO, ZrO₂, and SiO₂, carbideceramics such as SiC, TiC, WC, and B₄ C, nitride ceramics such as Si₃N₄, BN, and AlN, boride ceramics such as TiB₂ and ZrB₂, and silicideceramics such as MoSi₂, WSi₂, and CrSi₂. Molded articles of theseceramics have been hitherto prepared at a very high temperature. Inrecent years, a sintering assistant has been energetically studied forthe purpose of lowering the sintering temperature and the sinteringpressure. The sintering assistant serves to improve the sinterability ofceramics and, at the same time, to prevent the sinter particles fromgrowing, so that not only the formation of voids among the particles isprevented but also the grain boundaries are packed at a high density.

Examples of the sintering assistant used in the art include MgO, NiO,CaO, TiO₂, Al₂ O₃, Y₂ O₃, B₄ C, B, and C. These additives are selectedbecause they can bring about the occurrence of a phase reaction betweenthe base ceramic and the additive so as to promote the sintering of theceramic having a poor self-sinterability or because the sintering caneasily proceed due to the formation of a plasticized liquid phase by theadditive at a high temperature. Further, B and C can serve to enhancethe sinterability through a lowering in the surface energy of SiCcrystals.

However, when the above-described sintering assistants are present,there is a possibility that second and third phases are formed due tothe reaction of a base ceramic with an assistant. These phases arepresent mainly at the crystal grain boundaries, and constituents ofthese phases bring about plastic deformation when exposed to a hightemperature, which makes it impossible to produce a sinter havingexcellent high-temperature strength. For example, the addition of MgO toSi₃ N₄ brings about the formation of a vitreous phase comprising SiMgO₃.Since this fills up the grain boundaries, an increase in the density canbe attained. However, the mechanical strength of the sinter at a hightemperature is sharply lowered at about 1,000° C. due to the softeningof the vitreous phase. In order to avoid the abovedescribed lowering inthe strength at a high temperature, it is preferred to select anassistant which does not form any vitreous phase. However, this kind ofassistant is generally low in the ability of sintering, so that itbecomes impossible to produce a satisfactory molded material.

As a means for eliminating the above-described inconvenience, a proposalhas been made on a process for producing a ceramic sinter lesssusceptible to the lowering in the strength at a high temperaturewherein a particular organometallic polymer is used as a binder for aceramic powder and a mixture of the ceramic powder with the binder isheat sintered.

For example, U.S. Pat. Nos. 4,336,215 and 4,556,526 each disclosed aprocess for producing a sinter which comprises heat-sintering a mixtureof a polymetallocarbosilane with a ceramic powder after molding orsimultaneously with the molding.

In the process described in the above-described U.S. Patents, thepolymetallocarbosilane used as a binder of a ceramic powder is convertedinto an inorganic material when the mixture is heated at a hightemperature. Since this inorganic material is a substance having a highmelting point, the resultant sinter has relatively high strength even ata high temperature. This is because, as described on col. 6, lines 18 to31 of the U.S. Pat. No. 4,336,215, the sinter produced in the processdescribed in the above-described patents mainly comprises siliconcarbide particles, a solid solution composed of SiC and TiC eachproduced by thermal decomposition of polytitanocarbosilane, and a grainboundary phase mainly composed of TiC_(1-x).

With respect to the strength of sinters produced by the processesdescribed in the above-described patents, for example, a sinter having adeflective strength (bending strength) of 13.0 kg/mm² was produced inExample 7 of the U.S. Pat. No. 4,336,215 by molding a mixture of asilicon carbide powder with polytitanocarbosilane and sintering themolded material at 1,200° C., and a sinter having a deflective strength(bending strength) of 25.1 kg/mm² was produced in Example 11 of the sameU.S. Patent by preliminarily heating the above-described mixture at 600°C., grinding the heated mixture, and hot-pressing the ground mixture at1800° C.

In recent years, engineering ceramics have been required to have higherfunctions. For example, the development of a sinter which has highstrength and hardly brings about a lowering in the strength at a hightemperature has been desired.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a process for producinga high-strength and high-toughness sinter which has high mechanicalstrength and excellent heat resistance at room temperature, hardlybrings about a lowering in the strength even at a high temperature, andfurther has excellent toughness.

The above-described object of the present invention can be attained by ahigh-strength and high-toughness sinter comprising a crystal agglomeratemaintaining a fibrous shape and composed of crystals of SiC and MC_(1-x)wherein M is Ti and/or Zr and x is a number of 0 or more but less than 1(hereinafter referred to as the "first ceramic sinter").

Further, the above-described object of the present invention can beattained by a high-strength and high toughness sinter comprising flakyand/or acicular SiC crystals and an ultrafine grain agglomerate composedof SiC and and MC wherein M is Ti and/or Zr (hereinafter referred to asthe "second ceramic sinter").

The present invention also provides a preferable process for producingthe above-described first ceramic sinter, more specifically a processfor producing a high-strength and high-toughness sinter comprising acrystal agglomerate maintaining a fibrous shape and composed of crystalsof SiC and MC_(1-x) wherein M is Ti and/or Zr and x is a number of 0 ormore but less than 1, which comprises laminating an inorganic fibercomposed of the following inorganic material (i), (ii), or (iii) to forma laminate, molding the laminate into a predetermined shape, andconducting heat-sintering simultaneously with the molding or after themolding in an atmosphere comprising at least one member selected fromthe group consisting of a vacuum, an inert gas, a reducing gas, and ahydrocarbon gas at a temperature of 1,700 to 2,200° C.:

(i) an amorphous substance essentially consisting of silicon, M, carbon,and oxygen,

(ii) an agglomerate comprising fine crystalline grains each having adiameter of 500 Å or less and essentially consisting of β-SiC, MC, asolid solution composed of β-SiC and MC and/or MC_(1-x), wherein SiO_(y)and MO_(z), wherein O<y, z≦2, may be present around these ultrafinecrystalline grains, and

(iii) a system comprising a mixture of said amorphous substance (i) withsaid agglomerate (ii),

wherein M is Ti and/or Zr and X is a number of 0 or more but less than1.

Further, the present invention provides a preferable process forproducing the above-described second ceramic sinter, more specifically aprocess for producing a high-strength and high toughness sintercomprising flaky and/or acicular SiC crystals and an ultrafine grainagglomerate composed of SiC and MC, which comprises molding into apredetermined shape

either a mixture or a laminate prepared by mixing or laminating aninorganic fiber composed of the following inorganic material (i), (ii),or (iii) and a powder having the same composition as that of saidinorganic fiber or a powder of the following inorganic material (iv),(v), or (vi),

or a powder prepared by grinding an inorganic fiber composed of thefollowing inorganic material (i), (ii), or (iii), and conductingheat-sintering simultaneously with the molding or after the molding inan atmosphere comprising at least one member selected from the groupconsisting of a vacuum, an inert gas, a reducing gas, and a hydrocarbongas at a temperature of 1,700 to 2,200° C.:

(i) an amorphous substance essentially consisting of silicon, M, carbon,and oxygen, wherein M is Ti and/ or Zr and x is a number of 0 or morebut less than 1,

(ii) an agglomerate comprising fine crystalline grains each having adiameter of 500 Å or less and essentially consisting of β-SiC, MC, asolid solution composed of β-SiC and MC and/or MC_(1-x), wherein SiO_(y)and MO_(z), wherein O<y, z≦2, each in an amorphous and/or crystallineform may be present around these ultrafine crystalline grains,

(iii) a system comprising a mixture of said amorphous substance (i) withsaid agglomerate (ii),

(iv) an amorphous substance essentially consisting of Si, C, and O,

(v) an agglomerate comprising ultrafine crystalline grains each having adiameter of 500 Å or less and essentially consisting of β-SiC, whereinSiO_(y) in an amorphous and/or crystalline form may be present aroundthe ultrafine crystalline grains, and

(vi) a system comprising a mixture of said amorphous substance (iv) withsaid agglomerate (v).

The sinter of the present invention has high mechanical strength andexcellent heat resistance at room temperature, hardly brings about alowering in the strength even at a high temperature and further hasexcellent toughness, and the process of the present invention enablesthe above-described sinter to be produced on an industrial scale.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a surface reflection electron photomicrograph showing thestructure of a crystal of a ceramic sinter according to the presentinvention prepared in Example 1,

FIG. 2 is a further enlarged surface reflection electron photomicrographof FIG. 1,

FIG. 3 is a surface reflection electron photomicrograph showing thestructure of a crystal of a ceramic sinter according to the presentinvention prepared in Example 2, and

FIG. 4 is a surface reflection electron photomicrograph showing thestructure of a crystal of a ceramic sinter according to the presentinvention prepared in Example 4.

DETAILED DESCRIPTION OF THE INVENTION

At the outset, the first ceramic sinter of the present invention will bedescribed.

It is preferred that the crystal agglomerate maintaining a fibrous shapeand constituting the first ceramic sinter of the present inventioncomprises at least one member selected from among massive, flaky, andacicular SiC crystals, and crystals in the form of an ultrafine graincomposed of SiC and MC_(1-x), wherein M is Ti and/or Zr and x is anumber of 0 or more but less than 1. The term "massive" used herein isintended to mean preferably a mass having a side length of 1 to 20 μm inwhich grains are grown in the three-dimensional direction, the term"flaky" used herein is intended to mean preferably a scale having alength of 1 to 20 μm, and the term "acicular" used herein is intended tomean preferably a shape having a length of 1 to 20 μm and a length tothickness ratio of 1.5 to 20. The size of the above-described ultrafineparticle crystal is usually 500 Å or less.

It is preferred that the first ceramic sinter of the present inventioncontains 40% by weight of massive, flaky, or acicular SiC crystals. Whenthe amount of these crystals is too small, the strength of the sinter islowered. The upper limit of the amount of these crystals is usually 95%by weight.

Next, the process for producing the first ceramic sinter of the presentinvention will be described.

The inorganic fiber comprising the above-described inorganic material(i), (ii), or (iii) (hereinafter referred to as the "inorganic fiber[A]") used as the starting material in the process for producing thefirst ceramic sinter of the present invention exhibits highself-sinterability, which makes it possible to produce an excellentsinter through a treatment at 1,700 to 2,200° C. without addition of anysintering assistant.

The inorganic fiber [A] can be prepared by a process described in U.S.Pat. Nos. 4,342,712, 4,515,742 or the like. For example, said inorganicfiber [A] can be prepared by melt-spinning polytitanocarbosilane orpolyzirconocarbosilane, making the resultant fiber infusible through aheat treatment in the air, and baking the fiber in an inert gas at 800to 1,500° C.

The above-described inorganic fiber may be used in the form of acontinuous fiber, a chopped short fiber prepared by cutting a continuousfiber, a weave such as a plain weave, a three-dimensional weave, or anon-woven fabric prepared from a continuous fiber, and a sheet materialprepared by drawing and arranging a continuous fiber in one direction.

According to the present invention, crystals are preferentially grownwithin an inorganic fiber, so that it becomes possible to produce aceramic sinter which exhibits a crystal orientation reflecting the formof use of the fiber and has been deformed so as to fill up the gapsbetween the fibers most effectively. For example, the use of theabovedescribed plain weave as the inorganic fiber brings about theformation of a sinter wherein a crystal agglomerate maintaining afibrous shape is in the same oriented state as that of the plain weavelaminate, the use of the above-described sheet material as the inorganicfiber brings about a sinter wherein a crystal agglomerate maintaining afibrous shape is in the same oriented state as that of a sheet materiallaminate drawn in one direction, the use of the above-describedthree-dimensional weave as the inorganic fiber brings about a sinterwherein a crystal agglomerate maintaining a fibrous shape is in the sameoriented state as that of the three-dimensional weave, and the use ofthe above-described chopped short fiber as the inorganic fiber bringsabout the formation of a sinter wherein a crystal agglomeratemaintaining a fibrous shape is oriented at random. The above-describedsinters are each prepared in such a state that the section of thecrystal agglomerate maintaining a fibrous shape is deformed to have apolygonal shape and the crystal agglomerates are excellently linked orbonded to each other without the intervention of any matrix among them.

Accordingly, in the present invention, the ceramic sinter havingexcellent performance can be produced by preparing an inorganic fiberlaminate and conducting heat-sintering after molding of the laminateinto a desired shape or simultaneously with the molding.

The sintering can be conducted by a process wherein a laminate aftermolding is sintered under enhanced, atmospheric or reduced pressure or ahot press process wherein molding and sintering are simultaneouslyconducted.

In the above-described process which comprises conducting molding andsintering in a separate manner, molding is conducted by pressing thelaminate under a pressure of 100 to 5,000 kg/cm² through a mold pressprocess, a rubber press process, an extrusion process, or a sheetprocess to have a predetermined shape. In the above-described molding,if necessary, a starting material of the inorganic fiber, i.e.,polycarbosilane or polytitanocarbosilane, polyzirconocarbosilane, or acommercially available organic polymer may be used as a binder. Themolding prepared above is then sintered to give a ceramic sinter of thepresent invention.

When sintering is conducted by the hot press process, a mold made ofgraphite and sprayed with a releasing agent composed of BN is used, andthe laminate is pressed under a pressure of 2 to 2,000 kg/cm² withheating, thereby giving a sinter.

The heat-sintering temperature is 1,700 to 2,200° C., preferably 1,900to 2,100° C. The heating at this temperature brings about the formationof massive, flaky, and/or acicular SiC crystals, thus forming ahigh-strength and high-toughness ceramic sinter wherein the SiC crystalsare uniformly dispersed in an ultrafine grain agglomerate comprising SiCand TiC and/or ZrC. When the heat-sintering temperature is below 1,700°C., no massive, flaky, or acicular SiC crystal is formed, which makes itimpossible to produce a high-strength sinter. On the other hand, whenthe heat-sintering temperature is above 2,200° C., the formed SiCcrystals are apt to be decomposed. The heat-sintering is conducted in anatmosphere comprising at least one member selected from the groupconsisting of a vacuum, an inert gas, a reducing gas, and a hydrocarbongas. Examples of the inert gas include nitrogen and carbon dioxidegases, examples of the reducing gas include hydrogen and carbonmonooxide gases, and examples of the hydrocarbon gas include methane,ethane, propane, and butane gases.

The first ceramic sinter of the present invention exhibits much higherstrength at room temperature than that of conventional ceramic sinters,hardly brings about a lowering in the strength even at a hightemperature, and exhibits a fracture toughness value 2 to 10 timeshigher than that of the conventional ceramic sinters. The oxygen and atleast part of carbon in a nonstoichiometric amount contained in theinorganic fiber are released during the above-described sinteringaccording to the following reaction:

    2C+SiO→SiC+CO

    3C+SiO.sub.2 →SiC+2CO.

Presumably this brings about a lowering in the surface energy of the SiCgrain and thus improves the sinterability. However, the sinter maycontain 10% by weight, based on the sinter, of free carbon in anon-stoichiometric amount with respect to the silicon atom and M andfurther 15% by weight, based on the sinter, of oxygen in the form ofSiO_(y), wherein O<y≦2, and/or MO_(z), wherein O<z≦2.

The sinter may be produced also by, if necessary, impregnating theabove-described molding of the laminate before sintering with a startingmaterial of the inorganic fiber, i.e., polycarbosilane orpolytitanocarbosilane, polyzirconocarbosilane, or a silane couplingagent to treat the surface of the inorganic fiber constituting theabove-described laminate, preliminarily heating the treated laminate at800 to 1,500° C. in an atmosphere comprising at least one memberselected from the group consisting of a vacuum, an inert gas, a reducinggas, and a hydrocarbon gas, and sintering the heated laminate at 1,700to 2,200° C.

Next, the process for producing the second ceramic sinter of the presentinvention will be described.

When the above description with respect to the first ceramic sinter andthe process for producing the same is directly applicable to the secondceramic sinter and the process for producing the same, such descriptionis omitted in the following description according to need. Therefore,the above description applies where there is not detailed description oronly an insufficient description hereinbelow.

The ceramic sinter of the present invention comprises flaky and/oracicular SiC crystals and an ultrafine grain agglomerate composed of SiCand MC.

It is preferred that the ceramic sinter of the present invention containthe flaky and/or acicular SiC crystals in an amount of 40% by weight.When the amount of these crystals is too small, the strength of thecomposite material is lowered. Further, the upper limit of the amount ofthese crystals is usually 95% by weight.

The starting materials used in the process for producing the secondceramic sinter of the present invention, i.e., the inorganic fibercontaining silicon, carbon, and oxygen and comprising theabove-described inorganic material (iv), (v), or (vi) (hereinafterreferred to as the "inorganic fiber [B]"), the above-described inorganicfiber [A], and a powder prepared by grinding said inorganic fiber [A](hereinafter referred to as the "powder [A]") are all so highlysinterable that it is possible to produce a sinter through a treatmentat a temperature of 1,700 to 2,200° C. without addition of any sinteringassistant. In particular, the inorganic fiber [A] or a powder preparedby grinding said inorganic fiber [A] is highly sinterable, which makesit possible to produce an excellent sinter through heat-sintering at theabove-described temperature. When the diameter of the fine crystallinegrains constituting the above-described inorganic materials (ii) and(iv) exceeds 500 Å, the strength of the sinter is lowered.

The inorganic fiber [B] can be prepared by melt-spinning polycarbosilaneprepared according to a process described in Japanese Patent Laid-OpenNo. 126300/1976, 139929/1976, or the like, making the resultant fiberinfusible through a heat treatment in the air, and baking the treatedfiber at 800 to 1,500° C.

There is no particular limitation with respect to the form of use of theinorganic fiber [A] or [B], and as with the case of the first ceramicsinter, they may be used in the form of a continuous fiber, a choppedshort fiber prepared by cutting a continuous fiber, a weave such as aplain weave, a three-dimensional weave, or a non-woven fabric preparedfrom a continuous fiber, and a sheet material prepared by drawing andarranging a continuous fiber in one direction.

There is no particular limitation with respect to the powder used in theprocess for producing the second ceramic sinter of the present inventionas far as the powder has the same composition as that of the inorganicfiber [A] or [B], and examples of the powder include a ground product ofthe abovedescribed inorganic fiber and a ground product of a sinteredand molded material having the same composition as that of the inorganicfiber. For example, the inorganic fiber [A] can be ground into a powderwith a grinder known to the art, e.g., a ball mill, a vibrating mill, anattritor, or the like. The particle diameter of the powder is usually 1to 50 μm.

There is no particular limitation also with respect to the proportion ofthe inorganic fiber to the powder, and the inorganic fiber is usuallyused in an amount of 10 to 70% by weight based on the total amount ofthe inorganic fiber and the powder.

With respect to the combination of the inorganic fiber with the powder,it is necessary to use the inorganic fiber [A] or a ground productthereof as at least one of the fiber and the powder. Examples of thecombination include:

(1) a combination of the inorganic fiber [A] with a ground product ofsaid fiber [A],

(2) a combination of the inorganic fiber [A] with a ground product of asinter having the same composition as that of said fiber [A],

(3) a combination of the inorganic fiber [A] with the inorganic fiber[B],

(4) a combination of the inorganic fiber [A] with a ground product of asinter having the same composition as that of the inorganic fiber [B],and

(5) a combination of the inorganic fiber [B] with a ground product ofthe inorganic fiber [A].

It is preferred to mix the inorganic fiber and the powder uniformly witheach other. When the inorganic fiber is a chopped material, it may bemixed with the powder by making use of a mixer known to the art. Whenthe inorganic fiber is a long fiber, a woven fabric, or a sheetmaterial, the inorganic fiber layer and the powder layer may be put ontop of each other to prepare a laminate.

The second ceramic sinter of the present invention may also be producedby using only the above-described powder [A] as the starting material.Alternatively, said powder [A] may be used in combination with theinorganic fiber [B]. The inorganic fiber [B] has the same shape as thatof the inorganic fiber [A], and the particle diameter of the powder ofthe inorganic fiber [B] (hereinafter referred to as the "powder [B]")also is the same as that of the powder [A]. The powder [B] is used in anamount of generally 0 to 200 parts by weight, preferably 0 to 100 partsby weight based on 100 parts by weight of the powder [A].

The above-described mixture or laminate, or powder [A], alone or amixture of the powder [A] with the powder [B] is then heat-sinteredafter molding into a desired shape or simultaneously with the molding toproduce the second ceramic sinter of the present invention.

The sintering may be conducted by a process wherein the above-describedmixture, laminate, or powder is molded and then sintered under enhanced,atmospheric or reduced pressure, or a hot press process wherein moldingand sintering are simultaneously conducted.

In the process which comprises conducting molding and sintering in aseparate manner, molding and sintering may be conducted in the samemanner as those in the process of producing the first ceramic sinter.

When sintering is conducted by the hot press process, a mold made ofgraphite and sprayed with a releasing agent composed of BN is used, andthe mixture, laminate, powder [A] alone, or mixture of the powder [A]with the powder [B] is pressed under a pressure of 2 to 2,000 kg/cm²with heating, thereby giving a sinter.

The heat-sintering temperature is 1,700 to 2,200° C., preferably 1,900to 2,100° C. The heating at this temperature brings about the formationof massive, flaky, and/or acicular SiC crystals, thus forming ahigh-strength ceramic composite material wherein the SiC crystals areuniformly dispersed in an ultrafine grain agglomerate as a matrixcomprising SiC and MC. When the heat-sintering temperature is below1,700° C., neither massive nor acicular SiC crystal is formed, whichmakes it impossible to produce a high-strength composite material. Onthe other hand, when the heat-sintering temperature is above 2,200° C.,the formed SiC crystals or the matrix are apt to be decomposed.

The second ceramic sinter of the present invention exhibits much higherstrength at room temperature than that of conventional ceramic compositematerial, hardly brings about a lowering in the strength even at a hightemperature, and further exhibits a fracture toughness value 1.2 to 1.5times higher than that of the conventional ceramic sinters.

The oxygen and at least part of carbon in a non-stoichiometric amountcontained in the inorganic fiber are released during the above-describedsintering according to the following reaction:

    2C+SiO→SiC+CO

    3C+SiO.sub.2 →SiC+2CO.

Presumably this brings about a lowering in the surface energy of the SiCgrain and thus improves the sinterability. However, the sinter maycontain 10% by weight, based on the sinter, of free carbon in anon-stoichiometric amount with respect to the silicon atom and M andfurther 15% by weight, based on the sinter, of oxygen in the form ofSiO_(y), wherein O<y≦2, and/or MO_(z), wherein O<z≦2.

Further, if necessary, as in the case of the production of the firstceramic sinter, the sintering may be conducted after the molded materialof the powder is impregnated with the starting material of the inorganicfiber [A] or [A].

The above-described object of the present invention can be attained alsoby a high-strength ceramic sinter comprising flaky, massive, or acicularSiC crystals and an ultrafine grain SiC crystal agglomerate uniformlydispersed in each other. A preferable process of producing thishigh-strength ceramic sinter includes a process for producing a ceramiccomposite material comprising flaky, massive, and/or acicular SiCcrystals and an ultrafine grain SiC crystal agglomerate uniformlydispersed in each other, said process comprising molding into apredetermined shape

either a mixture or a laminate produced by mixing or laminating aninorganic fiber composed of the following inorganic material (iv), (v),or (vi) and a powder prepared by grinding said inorganic fiber or apowder prepared by grinding a sinter having the same composition as thatof said inorganic fiber,

or a filled material prepared by filling fiber gasp of a nonwoven fabricand/or a three-dimensional woven fabric each comprising an inorganicfiber composed of an inorganic material (iv), (v), or (vi).with a finepowder prepared by grinding an inorganic material having the samecomposition as that of said inorganic fiber,

conducting heat-sintering simultaneously with the molding or after themolding in an atmosphere comprising at least one member selected fromthe group consisting of a vacuum, an inert gas, a reducing gas, and ahydrocarbon gas at a temperature of 1,700 to 2,200° C.

The detail of the above-described ceramic sinter and the process forpreparing the same is identical to that of the above-described first andsecond ceramic sinters and the process for preparing the same andtherefore may be easily understood by referring to the abovedescription.

The present invention will now be described with reference to thefollowing Examples.

REFERENCE EXAMPLE 1

A 5-λ three-necked flask was charged with 2.5 λ of anhydrous xylene and400 g of sodium. The mixture was heated to the boiling point of xyleneunder a nitrogen gas stream, and 1 λ of dimethyldichlorosilane wasdropwise added thereto over a period of 1 hr. After the completion ofaddition, the mixture was heated under reflux for 10 hr to formprecipitates. The precipitates were collected by filtration and washedwith methanol and then with water to prepare 420 g of polydimethylsilanein the form of a white powder.

Separately, 759 g of diphenyldichlorosilane and 124 g of boric acid wereheated in n-butyl ether in a nitrogen atmosphere at a temperature of 100to 120° C. to prepare a white resinous material. The resinous materialwas further heated in vacuo at 400° C. for 1 hr to prepare 530 g ofpolyborodiphenylsiloxane.

8.27 g of the above-prepared polyborodiphenylsiloxane was mixed with 250g of the above-described polydimethylsilane. The mixture was heated to350° C. in a 2-λ quartz tube equipped with a reflux tube in a nitrogengas stream and polymerized for 6 hr. The reaction product was allowed tocool at room temperature. Xylene was then added thereto to withdraw thereaction product in the form of a solution. Xylene was evaporated, andthe residue was concentrated in a nitrogen gas stream at 320° C. for 1hr to prepare polycarbosilane which is a starting material of theinorganic fiber [B].

REFERENCE EXAMPLE 2

10 g of tetrabutoxysilane was added to 50 g of polycarbosilane preparedin Reference Example 1 which is a starting material of the inorganicfiber [B]. 40 ml of xylene was added to the mixture, and the mixture wasthen stirred in a nitrogen atmosphere at 130° C. for 1 hr. Thetemperature was slowly raised, and the mixture was polymerized at 320°C. for 2 hr to prepare polytitanocarbosilane which is a startingmaterial of the inorganic fiber [A].

REFERENCE EXAMPLE 3

Polycarbosilane prepared in Reference Example 1 which is a startingmaterial of the inorganic fiber [B] was melt-spun into a fiber. Thefiber was heated in the air to 180° C. at a temperature raising rate of20° C./hr to make the fiber infusible, heated in a nitrogen atmosphereto 1300° C. at a temperature raising rate of 200° C./hr, and maintainedat that temperature for 1 hr. The heat-treated fiber was allowed to coolto prepare the inorganic fiber [B].

The inorganic fiber [B] was ground in a mortar made of silicon nitrideto prepare the powder [B] of 200 mesh or less.

REFERENCE EXAMPLE 4

Polytitanocarbosilane prepared in Reference Example 2 which is astarting material of the inorganic fiber [A] was melt-spun into a fiber.The fiber was heated in the air to 180° C. at a temperature raising rateof 20° C./hr to make the fiber insufible, heated in a nitrogenatmosphere to 1300° C. at a temperature raising rate of 200° C./hr, andmaintained at that temperature for 1 hr. The heat-treated fiber wasallowed to cool to prepare the inorganic fiber [A].

The inorganic fiber [A] was ground in a mortar made of silicon nitrideto prepare the powder [A] of 200 mesh or less.

EXAMPLE 1

Polytitanocarbosilane prepared in Reference Example 2 was melt-spun intoa fiber. The fiber was heated in the air to 170° C. at a temperatureraising rate of 20° C./hr to make the fiber infusible, heated in anitrogen atmosphere to 1000° C. at a temperature raising rate of 200°C./hr, and maintained at that temperature for 1 hr. The heat-treatedfiber was allowed to cool to prepare an inorganic continuous fiber [A].

Plain woven fabrics each comprising the above-described inorganiccontinuous fiber [A] were put on top of each other. The laminate was setin a carbon die (a sheet material having a size of 3 mm×10 mm×10 mm),hot-pressed in an argon gas stream under a pressure of 600 kg/cm² at2,000° C. for 0.5 hr to produce the first ceramic sinter of the presentinvention.

The ceramic sinter of the present invention thus produced had a bendingstrength of 80 kg/mm² (at room temperature) and 76 kg/mm² (at 1400° C.)and a density of 3.0 g/cm³. Further, the ceramic sinter exhibited afracture toughness value (Kic: 24) 8 times higher than that of a ceramicsinter produced from only the powder without use of the plain wovenfabric.

The fracture of the above-described ceramic sinter was observed under asurface reflection electron microscope. As a result, it was found that,as shown in FIG. 1, a fibrous crystal agglomerate which had beendeformed into a polygonal form was closely packed in the most effectivemanner and the orientation of the plain woven fabric was maintained.

The above-described fibrous crystal agglomerate was further observedwith a larger magnification. As a result, it was found that,. as shownin FIG. 2, acicular or flaky crystals (SiC) were present.

EXAMPLE 2

Polytitanocarbosilane prepared in Reference Example 2 was melt-spun intoa fiber. The fiber was heated in the air to 170° C. at a temperatureraising rate of 20° C./hr to make the fiber infusible, heated in anitrogen atmosphere to 1000° C. at a temperature raising rate of 200°C./hr, and maintained at that temperature for 1 hr. The heat-treatedfiber was allowed to cool to prepare an inorganic continuous fiber [A].

Similarly, polycarbosilane prepared in Reference Example 1 was spun,made infusible, and heat-treated to form an inorganic continuous fiber[B]. The inorganic continuous fiber [B] was ground in a mortar made ofsilicon nitride to prepare a powder of 200 mesh or less. The powder andthe above-described inorganic continuous fiber [A] were put on top ofeach other. The laminate was set in a carbon die (a sheet materialhaving a size of 3 mm×10 mm×10 mm), hot-pressed in an argon gas streamunder a pressure of 600 kg/cm² at 2,000° C. for 0.5 hr to produce thesecond ceramic sinter of the present invention.

The proportion of the above-described plain woven fabric to theabove-described powder was 1 : 3.

The ceramic sinter of the present invention thus produced had a bendingstrength of 87 kg/mm² (at room temperature) and 84 kg/mm² (at 1400° C.)and a density of 3.0 g/cm³. Further, the ceramic sinter exhibited afracture toughness value 1.4 times higher than that of a ceramic sinterproduced from only the powder without use of the plain woven fabric.

The fracture of the above-described ceramic sinter was observed under asurface reflection electron microscope. As a result, it was found that,as shown in FIG. 3, acicular or flaky crystals (SiC) were present.

EXAMPLE 3

Polytitanocarbosilane prepared in Reference Example 2 was melt-spun intoa fiber. The fiber was heated in the air to 170° C. at a temperatureraising rate of 20° C./hr to make the fiber infusible, heated in anitrogen atmosphere to 1200° C. at a temperature raising rate of 200°C./hr, and maintained at that temperature for 1 hr. The heat-treatedfiber was allowed to cool to prepare an inorganic continuous fiber [A].A powder of 200 mesh or less prepared by grinding the inorganiccontinuous fiber [A] in a mortar made of silicon nitride and a plainwoven fabric comprising the above-described continuous fiber [A] wereput on top of each other. The laminate was set in a carbon die (a sheetmaterial having a size of 3 mm×10 mm×10 mm), hot-pressed in an argon gasstream under a pressure of 700 kg/cm² at 2,000° C. for 0.5 hr to producethe second ceramic sinter of the present invention.

The proportion of the above-described plain woven fabric to theabove-described powder was 1 : 3.

The ceramic sinter of the present invention thus produced had a bendingstrength of 100 kg/mm² (at room temperature) and 95 kg/mm² (at 1400° C.)and a density of 3.1 g/cm³. Further, the ceramic sinter exhibited afracture toughness value 1.5 times higher than that of a ceramic sinterproduced from only the powder without use of the plain woven fabric.

EXAMPLE 4

The powder [A] prepared in Reference Example 4 was set in a carbon die(a sheet material having a size of 3 mm×10 mm×10 mm), hot-pressed in anargon gas stream under a pressure of 700 kg/cm² at 2,100° C. for 0.5 hrto produce the second ceramic sinter of the present invention.

The fracture of the above-described ceramic sinter was observed under asurface reflection electron microscope. As a result, it was found that,as shown in FIG. 4, acicular crystals were dispersed. The ceramic sinterhad a bending strength of 80 kg/mm² at room temperature and 74 kg/mm² at1400° C. Further, the ceramic sinter exhibited a fracture toughnessvalue 1.3 times higher than that of the conventional SiC sinter.

EXAMPLE 5

A powder prepared by mixing the powder [A] prepared in Reference Example4 with the powder [B] prepared in Reference Example 3 in a weight ratioof 1 : 1 was treated in the same manner as that of Example 1 to producethe second ceramic sinter of the present invention. The ceramic sinterthus produced had a bending strength of 75 kg/mm² at room temperatureand 69 kg/mm² at 1400° C. Further, the ceramic sinter exhibited afracture toughness value 1.2 times higher than that of the conventionalSiC sinter.

EXAMPLE 6

Polycarbosilane prepared in Reference Example 1 was melt-spun into afiber. The fiber was heated in the air to 170° C. at a temperatureraising rate of 20° C./hr to make the fiber infusible, heated in anitrogen atmosphere to 1000° C.-at a temperature raising rate of 200°C./hr, and maintained at that temperature for 1 hr. The heat-treatedfiber was allowed to cool to prepare an inorganic continuous fiber [B].

A powder of 200 mesh or less prepared by grinding the inorganiccontinuous fiber [B] in a mortar made of silicon nitride and a plainwoven fabric comprising the above-described continuous fiber were put ontop of each other. The laminate was set in a carbon die (a sheetmaterial having a size of 3 mm×10 mm×10 mm), hot-pressed in an argon gasstream under a pressure of 700 kg/cm² at 2,000° C. for 0.5 hr to producea ceramic sinter.

The proportion of the above-described plain woven fabric to theabove-described powder was 1 : 3.

The ceramic sinter thus produced had a bending strength of 76 kg/mm² (atroom temperature) and 73 kg/mm² (at 1400° C.) and a density of 3.0g/cm³. Further, the ceramic sinter exhibited a fracture toughness value1.4 times higher than that of a ceramic sinter prepared from only thepowder without use of the plain woven fabric.

EXAMPLE 7

Polycarbosilane prepared in Reference Example 1 was melt-spun into afiber. The fiber was heated in the air to 170° C. at a temperatureraising rate of 20° C./hr to make the fiber infusible, heated in anitrogen atmosphere to 1200° C. at a temperature raising rate of 200°C./hr, and maintained at that temperature for 1 hr. The heat-treatedfiber was allowed to cool to prepare an inorganic continuous fiber [B].

A powder of 200 mesh or less prepared by grinding the inorganiccontinuous fiber [B] in a mortar made of silicon nitride and a plainwoven fabric comprising the above-described continuous fiber were put ontop of each other. The laminate was set in a carbon die (a sheetmaterial having a size of 3 mm×10 mm×10 mm), hot-pressed in an argon gasstream under a pressure of 600 kg/cm² at 2,000° C. for 0.5 hr to producea ceramic sinter.

The proportion of the above-described plain woven fabric to theabove-described powder was 1 : 3.

The ceramic sinter thus produced had a bending strength of 68 kg/mm² (atroom temperature) and 63 kg/mm² (at 1400° C.) and a density of 3.1g/cm³. Further, the ceramic sinter exhibited a fracture toughness value1.3 times higher than that of a ceramic sinter produced from only thepowder without use of the plain woven fabric.

What is claimed is:
 1. A process for producing a high-strength and hightoughness sinter comprising a crystal agglomerate which maintains afibrous shape in the sinter and is composed of crystals of SiC andMC_(1-x) wherein M is Ti and/or Zr and x is a number of 0 or more butless than 1, wherein sections of at least part of said crystalagglomerate maintaining the fibrous shape are deformed as will be ableto most closely pack to fill up gaps which occur in said crystalagglomerate maintaining the fibrous shape, said processcomprisinglaminating inorganic fibers composed of inorganic material(i), (ii), or (iii) defined below, to form a laminate, molding thelaminate into a predetermined shape, and conducting heat-sinteringsimultaneously with the molding or after the molding in an atmospherecomprising at least one member selected from the group consisting of avacuum, an inert gas, a reducing gas, and a hydrocarbon gas at atemperature of 1,700 to 2,200° C.;(i) an amorphous substance consistingessentially of silicon, M, carbon, and oxygen, (ii) an agglomeratecomprising fine crystalline grains each having a diameter of 500 Å orless and essentially consisting of β-SiC, MC, a solid solution composedof β-SiC and MC and/or MC_(1-x) wherein SiO_(y) and MO_(z), wherein O<y,z≦2, may be present around these fine crystalline grains, and (iii) asystem comprising a mixture of said amorphous substance (i) with saidagglomerate (ii); and wherein said inorganic fibers are woven into aplain weave.
 2. The process for producing a high-strength and hightoughness sinter according to claim 1 wherein the sections of at leastpart of the fibers of said crystal agglomerate are deformed into apolygonal shape.
 3. The process of claim 1, wherein said inorganicmaterial is (i) an amorphous substance consisting essentially ofsilicon, M, carbon, and oxygen.
 4. The process of claim 1, wherein saidinorganic material is (ii) an agglomerate comprising fine crystallinegrains each having a diameter of 500 Å or less and consistingessentially of β-SiC, MC, a solid solution composed of β-SiC and MCand/or MC_(1-x) wherein SiO_(y) and MO_(z), wherein O<y, z≦2, may bepresent around these fine crystalline grains.
 5. The process of claim 1,wherein said inorganic material is (iii) a system comprising a mixtureof said amorphous substance (i) with said agglomerate (ii).